(1) Field of the Invention
The present invention relates to a method of heating a piping arrangement in which a pipe is heated through a high frequency wave induction and a heating coil to be used for the heating process, and more particularly, to a method of heating a piping arrangement and a heating coil which are applicable to the case where a configuration of the heating coil is sharply changed in such case that the heating coil is provided with stepped portions or there is a welded portion between a primary pipe and a pipe base at which the piping arrangement is greatly changed in cross section.
(2) Description of the Prior Art
In a piping arrangement for use with atomic plants, thermal plants, chemical plants and the like, pipes such as a straight pipe, a curved pipe, and other pipes are welded to each other to form a long piping. However, recently, for instance, an atomic plant has been used at a higher temperature and a higher pressure. Therefore, there is a great fear that a stress generated in such a piping would approach a yield stress of the material. Also, pipes are manufactured through plasticity machining and a welding technique is applied to the pipes to form a piping arrangement. A residual tension stress generated in the piping arrangement through such processes will be amplified with a repeated stress, which is generated upon operating the plant, e.g., a repeated thermal stress. In particular, when corrosive fluid is made to flow in the pipe, a corrosion fatigue must be considered. To prevent a stress-corrosion cracking in such a piping arrangement, the Japanese Patent Application Laid-Open No. 38246/78 discloses a heat processing method for pipe arrangement, in which coolant is provided in the pipe, the pipe is heated to thereby generate a difference in temperature between an outer surface and an inner surface of the pipe, and the inner surface is subjected to a yield tension stress whereas the outer surface is subjected to a yield compression stress. In that thermal processing method, under the condition that coolant, for instance, cooling water for a nuclear reactor in the case of the atomic energy plant be supplied in the pipe, a heating coil is wound on the outer surface of the pipe in the vicinity of the welded portion of the pipe. A magnetic field is generated by the application of current to the coil. The outer periphery of the pipe is then heated but the interior of the pipe is not so heated at a high temperature because of the coolant. As a result, a difference in temperature exists between the outer and inner surfaces of the pipe. It is preferable that the extent of such a temperature difference be such that during heating, compression yield is applied to the outer surface whereas tension yield is applied to the inner surface. Thus, when the heat processing is finished, the temperature of the pipe is gradually decreased so that as reversed to the phenomenon upon heating, the outer surface of the pipe is subjected to a residual tension stress whereas the inner surface thereof is subjected to a residual compression stress. Since the inner surface is improved by the compression stress thereby removing a cause of the stress-corrosion cracking, a resistance of the pipe against the corrosive fatigue is remarkably enhanced. Namely, in the atomic energy plant, the pipe may withstand a repeated thermal stress generated by the flow of cooling water through the pipe during the operation of the nuclear reactor, and a stress-corrosion cracking of the pipe may be prevented even under the existence of corrosive fluid.
Although such a heat processing method may be applied to pipe arrangements of various plants, there is a maximum temperature limit below which the heating is possible. For example, in case of stainless steel used in the atomic energy plant, the allowable maximum temperature for the outer surface of the pipe is approximately 550.degree. C. due to a sensitivity problem in excess accompanying the heating. In order to generate a yield compression stress on the outer surface of the pipe and a yield tension stress on the inner surface thereof within the upper limit, an important task to be solved is how the upper limit of the heating temperature is suppressed while keeping a necessary temperature difference large. To meet this requirement, it is necessary to keep the temperature possibly uniform in the pipe outer surface under the influence of the heating coil, without such localization of temperature that some parts are extremely heated and other parts remain at an extremely low temperature.
In the case where the configuration of the coil is simple, when the outer surface of the pipe is heated by the heating coil, a uniform temperature distribution may readily be obtained on the basis of a theory. However, if the configuration of the coil is complicated, a magnetic flux distribution generated by current flowing through the coil is changed so that it results in nonuniform distribution of temperature.
As an example in which nonuniform distribution in temperature is caused, the magnetic flux distribution is changed by the provision of stepped portions in the coil. A coil to be used for induction heating of the pipe has a large diameter. Moreover, it is very difficult to wind it in a spiral manner in a particular shape thereof in cross section. For this reason, one turn of the coil is included in substantially the same plane, and both the turn ends are bent in the opposite direction at a predetermined angle, respectively, and are connected to the adjacent turn ends thereby forming a single coil as a whole. The connecting portion of each adjacent two ends of the turns will be referred to as the stepped portion, which is different in magnetic flux than other portions. Therefore, the outer surface of the pipe confronted with the stepped portion is a place where it is difficult to increase a temperature.
As another example in which nonuniform distribution in temperature is caused, the configuration of the coil is complicated in the vicinity of a welded portion against a pipe base. More specifically, to couple a branched pipe or a nozzle to a primary pipe with the branched pipe or the nozzle being perpendicular to the primary pipe, this makes the piping arrangement per se complicated, as a result of which the coil configuration becomes complicated. Also in this case, a portion where a temperature is increased with difficulty will be generated by the variation in magnetic flux distribution.